It is known that actinic radiation, particularly in the near ultraviolet region, has a deleterious affect on both the appearance and properties of many organic materials and particularly organic polymers. For example, it is well known that normally colorless or light-colored polyesters will yellow on exposure to sunlight as do cellulosics such as cellulose acetate. Organic polymers of various types have been used extensively to prepare films, sheets, and various molded, shaped or formed articles. In order for these articles to be used successfully in outdoor applications where they are exposed to sunlight, they must be able to retain their appearance, particularly with respect to color, gloss, transparency, shape and various physical properties such as impact strength, tensile strength, etc. Such articles, however, tend to degrade, and the physical properties of the articles are reduced when they are exposed to outdoor conditions including ultraviolet light, heat, water and oxygen over extended periods of time thereby rendering such articles less useful for many outdoor applications. Accordingly, considerable effort has been expended in the industry to improve the ability of such polymer materials to resist ultraviolet degradation. Many suggestions have been made in the prior art to provide some degree of protection against discoloration and degradation of such organic materials by the addition of various stabilizing compounds including a variety of hindered amine light stabilizers, a number of antioxidants based on phenols, and various combinations and mixtures of such stabilizing compounds.
For example, the use of various hindered amine light stabilizers for stabilizing organic materials such as polyesters and polyurethanes are described in U.S. Pat. Nos. 4,208,522; 4,315,848; 4,511,596; and 4,524,165. Various hindered phenols have been suggested as stabilizing additives to organic polymers for improving the resistance to ultraviolet light degradation in, for example, U.S. Pat. Nos. 3,799,990; 4,208,522; and 4,524,165.
It also has been suggested in the prior art that polymeric materials can be stabilized against ultraviolet light deterioration by the use of various benzotriazole derivatives. Examples of patents describing various benzotriazole derivatives as stabilizers and ultraviolet absorbers include U.S. Pat. Nos. 3,004,896; 4,315,848; 4,511,596; and 4,524,165.
The use of formamidines as ultraviolet absorbing compounds also is described in the prior art. U.S. Pat. Nos. 4,021,471 and 4,085,062 describe a number of N,N'-bis-aromatic formamidines reported to be useful for the protection of materials against the degradative effects of ultraviolet light.
A number of patents have suggested the use of combinations of the known ultraviolet light stabilizers provides improved results. For example, U.S. Pat. No. 4,208,522 describes the use of a combination of certain hindered amine light stabilizers with antioxidants such as the hindered phenols. A variety of useful phenolic compounds are described in the U.S. Pat. No. 4,208,522. Combinations of hindered amine light stabilizers with benzotriazole derivatives are described in U.S. Pat. Nos. 4,315,848 and 4,511,596 describes combinations of hindered amine light stabilizers with a variety of ultraviolet absorbers including benzotriazole derivatives, hydroxybenzophenones, esters of benzoic acids, oxalic acid diamides, etc. It is suggested in U.S. Pat. No. 4,524,165 that polyesterether compositions can be stabilized by the combination of (1) at least one hindered phenolic antioxidant, (2) at least one ultraviolet light stabilizing compound similar to the ultraviolet absorbers disclosed in U.S. Pat. No. 4,511,596, and (3) at least one hindered amine light stabilizing compound of certain specified formulae. In spite of all the suggestions in the prior art for improving the stability of various organic materials to the degradative effects of ultraviolet light, there continues to be a need for improved stabilizer systems or organic materials, particularly organic polymers which will provide further improved properties with respect to color retention, gloss retention, transparency, shape, weatherability, etc.